Hydrogen sulfide pretreatment of lignocellulosic materials in alkaline pulping processes

ABSTRACT

DIRECTION OF THE SUPPORT TUBE (8) AND WHICH WHILE THE REACTOR IS IN OPERATION IS SUPPORTED ON THE LOCKING PLATE (5,6), AND WHILE THE REACTOR IS SHUT DOWN IS SUPPORTED ON THE SLEEVE TUBE (3) BY MEANS OF A PAWL DEVICE (14), THE LENGTH OF TRAVEL OF THE AUXILIARY TUBE (12) BEING SUCH THAT WHEN THE PAWL DEVICE IS IN OPERATION THE SUPPORT TUBE (8) IS AT LEAST PARTLY RELIEVE OF THE WEIGHT OF THE MODERATOR COLUMN.   1. IN AN ALKALINE PULPING PROCESS IN WHICH LIGNOCELLULOSIC MATERIAL IS PRE-TREATED WITH HYDROGEN SULFIDE, IN WHICH THE PRE-TREATED MATERIAL IS DIGESTED UNDER HEAT AND PRESSURE IN AN ALKALINE DIGESTING LIQUOR COMPRISING A SODIUM BASED DIGESTING AGENT TO FORM THE PULP, IN WHICH THE DIGESTED PULP IS SEPARATED FROM THE DIGESTING LIQUOR AFTER DIGESTION, AND IN WHICH THE DIGESTING LIQUOR IS PROCESSED FOR RECOVERY OF THE ALKALINE DIGESTING AGENT, THE IMPROVEMENT WHICH COMPRISES: PROVIDING FROM THE DIGESTING LIQUOR PROCESSED FOR RECOVERY OF THE ALKALINE DIGESTING AGENT, A HIGH SULFIDITY LIQUOR CONTAINING AT LEAST ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF SODIUM SULFIDE AND SODIUM HYDROSULFIDE AND HAVING A SULFIDITY OF ABOUT 75% TO 200% AND A WEIGHT RATIO OF SULFUR TO SULFUR PLUS SODIUM OF FROM ABOUT 0.35-0.58; CONTACTING THE LIGNOCELLULOSIC MATERIAL WITH SAID HIGH SUFIDITY LIQUOR IN A PRE-TREATMENT VESSEL PRIOR TO ALKALINE DIGESTION; INTRODUCING CARBON DIOXIDE GAS INTO SAID HIGH SULFIDITY LIQUOR IN SAID VESSEL, IN AN AMOUNT IN EXCESS OF STOICHIOMETRIC TO GENERATE HYDROGEN SULFIDE FROM SAID HIGH SULFIDITY LIQUOR; MAINTAINING THE TEMPERATURE IN SAID PRE-TREATMENT VESSEL WITHIN THE RANGE OF ABOUT 120 TO 165*C. FOR ABOUT 20 TO 200 MINUTES AND UNDER A PARTIAL PRESSURE OF HYDROGEN SULFIDE GAS OF ABOUT 10 TO 80 P.S.I. TO EFFECT REACTION BETWEEN HYDROGEN SULFIDE AND THE LIGNOCELLULOSIC MATERIAL IN SAID PRE-TREATMENT VESSEL TO INCREASE PULP YIELD OVER THAT OBTAINED BY AN ALKALINE PULPING PROCESS IN WHICH THE LIGNOCELLULOSIC MATERIAL IS DIGESTED IN SAID ALKALINE DIGESTING LIQUOR; SEPARATING PRE-TREATED LIGNOCELLULOSIC MATERIAL FROM THE ACCORDING TO WHICH THE SUPPORT TUBES PASS THROUGH THE BOTTOM OF THE REACTOR VESSEL AND ARE PROVIDED AT THE OUTER END WITH AT LEAST ONE PLUG WHICH ACTS AS A RADIATION SHIELD AND A COOLING-GAS SEAL, IN WHICH THE SUPPORT TUBE (8) IS FIXED IN A SLEEVE TUBE (3) BY MEANS OF A LOCKING PLATE (5, 6) SUCH THAT BETWEEN THE SUPPORT TUBE (8) AND THE SLEEVE TUBE (3) THERE IS AN ANNULAR GAP (11) CONTAINING AN AUXILIARY TUBE (12) WHICH IS MOVABLE IN THE AXIAL

Oct. 15, 1974 Filed- May 21, 1973 A. R. PROCTER ET L HYDROGEN SULFIDEPRETREATNENT F LIGNOCELLULOS MATERIALS IN ALKALINE PULPING PROCESSES 3Sheets-Sheet 2 f 5 THE COMPOSITION OF THE SULFIDE, HYDROSULFIDE, H28SYSTEM AND THE CARBONATE BICARBONATE SYSTEM As A FUNCTION OFPH.

s E 0.6- v 3 E 0.4 0.2 X

o 2 4 6 8 I0 I2 I4 p THE EFFECT OF FEED LIQUOR CHEMICAL COMPOSITION A?-To THE PRETREATMENT VESSEL 0N H28 PARTIAL PRESSURE AT EQUILIBRIUM IN ACONTINUOUS SYSTEM 06 FEED LIQUOR %s APPLIED ON WOOD I I 8 Lu L z 0.5 2 2k D K lg E fly Y Q (I) H 8 PARTIAL 'PRESSURE AT [40C Oct. 15, 1974 FiledMay 21. 1973 A. R. PROCTER E AL HYDROGEN SULFIDE PRETREATMENT 0FLIGNOCELLULOSIC MATERIALS IN ALKALINE PULPING PROCESSES 3 Sheets-Sheet J4 THE EFFECT OF H25 PARTIAL PRESSURE 0N PULP YIELD INCREASE PRETREATMENTCONDITIONS, OF /4oc,5o MIN.

S/S+Na WT. FRACTION IN FEED THE EFFECT OF FEED LIQUOR CHEMICALCOMPOSITION TO THE PRETREATMENT VESSEL ON CO AT EQUILIBRIUM C OMS UMPTION 00 CONSUMPTION, 0N WOOD United States Patent rm. (:1. D218 3/00,11/12 US. Cl. 162-30 Claims ABSTRACT OF THE DISCLOSURE A pretreatment oflignocellulosic material, such as wood chips, prior to digestion in analkaline pulping process, such as the kraft pulping process and thealkaline sulfite pulping process, is performed with hydrogen sulfidewhich is generated in the pretreatment by mixing a sulfide which iscompatible with the pulping process liquors and carbon dioxide gas. Thepretreated lignocellulosic materials are separated from the excesspretreatment chemicals and then pulped in the usual manner in a kraft oralkaline sulfite pulping process.

This is a continuation-in-part of application Ser. No. 276,853, filedJuly 31, 1972, now abandoned.

This invention relates to improved hydrogen sulfide pretreatment inalkaline pulping processes, and particularly the kraft pulping andalkaline sulfite pulping processes.

The present process is an improvement over the process of US. Pat.3,520,773, to Vinje and Worster. A process for increasing the pulp yieldby pretreating lignocellulosic material with hydrogen sulfide isdescribed in this patent. According to the patent, a significantincrease in pulp yield (6 to 7% wood) is obtainable if lignocellulosicmaterial, such as wood chips, sawdust and the like, is pretreated withhydrogen sulfide under heat and pressure in the presence of an alkalinebufier which is compatible with the pulping liquors and sufficient tomaintain the pH of the pretreatment medium slightly alkaline. Thepretreated lignocellulosic material is pulped by the regular kraft, sodaor alkaline sulfite process under conventional conditions.

In the prior Vinje and Worster process, the hydrogen sulfide makeuprequirement generally ranges from 1.0 to 1.5% on dry wood, particularlyWhen applied to softwoods. This makeup hydrogen sulfide indicates sulfurlosses that cannot be tolerated under existing and anticipated futurepollution abatement regulations, unless the process is combined withanother process that regenerates pure hydrogen sulfide from millchemicals. Processes that are presently available for hydrogen sulfideregeneration are expensive and complex.

The present process has the following advantages over the process ofPat. 3,520,773:

(1) No external production of pure H 8 is required, eliminatingexpensive, awkward gas handling systems.

(2) The overall process is simpler and completely self contained,requiring no external H S recovery system in the mill.

(3) H 8 gas is confined within a single vessel giving a safer processoperation.

(4) The pretreatment process can be applied to a oneline mill withoutcomplex sodium and sulfur balance problems.

3,841,962 Patented Oct. 15, 1974 The present process is an improvementin alkaline pulping processes wherein lignocellulosic material isdigested under heat and pressure in an alkaline digesting liquor. Theimprovement comprises pretreating the lignocellulosic material before itis contacted with the alkaline digesting liquor with hydrogen sulfidegenerated in the pretreatment by mixing a high sulfidity liquor which iscompatible with the liquors of the alkaline pulping process and anexcess of carbon dioxide. The excess of the carbon dioxide acts as abuffer. The pretreatment is conducted under conditions of temperatureand time and a stable partial pressure of hydrogen sulfide that effectreaction of the hydrogen sulfide with the lignocellulosic material andprovide higher pulp yields than obtained by conventional alkalinepulping process alone, calculated on the dry weight of thelignocellulosic material. The lignocellulosic material is preferablywood chips, but it may consist of annual plants, sawdust and the like.The hydrogen sulfidecarbon dioxide mixture is preferably sufficient tomaintain the pH of the pretreatment liquor in the neutral range.

The above outlined process is improved when the high sulfidity feedliquor solution contains sodium sulfide and/ or sodium hydrosulfide,together with sodium carbonate, sodium bicarbonate and sodium hydroxide.

The improvement according to this invention consists of generating therequired amount of hydrogen Sulfide in a pretreatment vessel by mixing asuitable high sulfidity liquor with an excess of carbon dioxide. Sodiumsulfide is preferred, but other sulfides such as potassium sulfide orammonium sulfide can be used. It has been found advan tageous to allowpart of the generated hydrogen sulfide gas to react with thelignocellulosic material and another part thereof to be recycledtogether with unused carbon dioxide gas. The makeup flow of carbondioxide and the selected high sulfidity liquor in a continuous system isregulated so as to provide a stable partial pressure of hydrogen sulfidesufficient for a 5 to 6% pulp yield increase calculated on the drylignocellulosic material. An advantage of this invention is that thepretreatment process can be applied to any kraft or alkaline sulfitepulping mill without upsetting the sodium and sulfur material balancesof that mill.

A significant increase in pulp yield is obtained if the lignocellulosicmaterial, such as wood chips, is pretreated in a suitable vessel with ahydrogen sulfide-carbon dioxide gas mixture under heat and pressure inthe presence of a high sulfidity liquor which is compatible with thepulping liquors of the kraft pulping process or of the alkaline sulfitepulping process. The pressure of the H S/CO gas mixture in the vessel iskept at a level sufficient to maintain the pH of the pretreatment liquorin the neutral range, that is, approximately 6 to 7.5. The pretreatedlignocellulosic material is then removed from the pretreatment vesseland pulped by the regular kraft or alkaline sulfite pulping processunder conventional conditions.

Examples of this invention are illustrated in the accompanying drawings,in which FIG. 1 diagrammatically illustrates the process for a kraftpulp mill,

FIGS. 2 to 5 are graphs illustrating various aspects of this invention.

In the process illustrated in FIG. 1, the sulfide containing liquor usedis kraft green liquor, which is essentially a mixture of sodiumcarbonate and sodium sulfide in a 3 to 1 to 2.5 to 1 mole ratio, thisbeing the equivalent to a 25% to 28.6% sulfidity green liquor. Thepretreatment takes place in a pretreatment vessel 10. It is important tomaintain the sulfidity of the makeup pretreatment liquor directed intothe vessel at 11 at a very high level, at least above and it can be ashigh as 200%. If the liquor sulfidity is not at a high level, theefilciency of the in situ hydrogen sulfide generation is impaired. Highsulfidity makeup pretreatment liquor can be prepared by evaporatinggreen liquor in a crystallizer/evaporator 14, which causes precipitationof sodium carbonate which is discharged at so as to give a maximum levelof about 95% sulfidity in the evaporated green liquor which is directedto the pretreatment vessel. This high sulfidity liquor may also beprepared by selective dissolution of sodium sulfide in the smelt in thedissolving tank 17 by using water or spent pretreatment liquor, sincesodium carbonate is less soluble than sodium sulfide. The concentratedhigh sulfidity liquor may be diluted with water or spent pretreatmentliquor prior to charging the pretreatment vessel. It has been found,however, that the best results are obtained when the required makeupliquor for the pretreatment stage is prepared by first precarbonatingthe green liquor with a carbon dioxide containing gas to reduce the pHfrom about 13.5 to about 12, although good results are attained with thepH within 10 and 13. At this pH level the green liquor contains amaximum of sodium hydrosulfide and a minimum of sodium bicarbonate (seeFIG. 3. This is important since maximum amounts of sodium hydrosulfideare required to generate maximum partial pressures of hydrogen sulfidein the pretreatment vessel, and minimum quantities of sodium bicarbonateare required to avoid increases in lime demand in the recausticizingpart of the mill recovery system. The precarbonated liquor is thensubjected to an evaporation/crystallization step whereby essentially allthe sodium carbonate is removed by crystallization. The concentratedsupernatent liquor, which contains sodium hydrosulfide, sodium sulfideand residual sodium carbonate and bicarbonate is then used as a feedliquor to the pretreatment vessel.

The hydrogen sulfide-carbon dioxide gas mixture is directed into vessel10. This gas is preferably recycled from a previous pretreatment in acontinuous system, together with makeup carbon dioxide from a gaspurifier 21. The gas pressure in vessel 10 preferably ranges from 120 to250 psi. Reaction of the makeup sodium sulfide and carbon dioxideresults in the generation of more hydrogen sulfide by the followingreactions:

From these equations it can be seen that the hydrogen sulfideconcentration is controlled by the chemical ratio [NaSH]/[NaHCO Thisratio is in turn controlled by the sulfur to sodium ratio in the feedliquor to the pretreatment vessel. The relationship between generatedhydrogn sulfide partial pressure (or overpressure) and the feed chemicalcomposition expressed as a ratio of weight of sulfur upon total weightof sulfur plus sodium, is shown in FIG. 3. To sustain eifectivepretreatment conditions, sufiicient to maximize the kraft pulp yieldincrease with Western hemlock wood chips, it has been found that an H Spartial pressure of at least 23 psi. measured at 140 C. is required(FIG. 4). However, partial pressures ranging from 10 to about 80 p.s.i.can be used with variations in the temperature. Typical pretreatmentconditions are 140 C. for 50 minutes, with a total pressure of about 180p.s.i.g. in the pretreatment vessel. Here again, the temperature canvary from 120 to 165 C. and the time from 200 down to minutes. A studyof FIG. 4 confirms the lower yield increases that are obtained in thepresence of low H 8 partial pressures. These conditions arise whenlignocellulosic material is treated with NaSH or Na s alone, asspecified in prior art, where no gas recovery or recycling is specifiedso as to maintain a stable partial pressure of H 8 gas.

Typical pretreatment conditions used with this system involve a sulfurconsumption by the lignocellulosic material of about 1.7% as sulfur onwood. This sulfur is organically bound to the liguocellulosic material.Depending on the efficiency of the washing in zone 24 of vessel 10 afterthe pretreatment, additional sulfur in the form of sodium hydrosulfidein the entrained pretreatment liquor may be carried over to the kraftstage with the pretreated chips which are directed at 26 to the kraftdigester 28. To maintain an overall sulfur balance in a one-line kraftmill, it is important that this total sulfur carry-over not exceed about2.5%, which is the normal amount of available active sulfur (as sodiumsulfide) in the green or white liquor of a kraft mill.

From the above equations and FIG. 3, it is obvious that a liquor with ahigh i S+Na ratio should be used for makeup and that sodium bicarbonatemust not be allowed to build up to a high level during pretreatmentsince this would shift the chemical equilibrium resulting in a lowerhydrogen sulfide partial pressure. To minimize the sulfur carryover tothe kraft stage, the pretreated chips are washed with water or degassedsulfur lean spent pretreatment liquor in zone 24. Spent pretreatmentliquor is removed from the pretreatment vessel and flashed in tank 30 torecover dissolved hydrogen sulfide and carbon dioxide gases. The spentpretreatment liquor is directed by 31 to tank 17 to dissolve the smeltwhich comes from the kraft recovery furnace 33. Alternatively, the spentpretreatment liquor can be used as a wash liquor in the pretreatmentvessel.

The crystallized sodium carbonate can be dissolved in tank 35 in weakwash, a dilute kraft mill solution consisting of sodium hydroxide-sodiumsulfide, to form a low sulfidity green liquor. This solution is thenrecausticized at 37 in the normal manner using lime from kiln 38-, toproduce a low sulfidity white liquor which is directed by 39 into thekraft mill digester 28. In the following pulping stage, the sulfurassociated in both organic and inorganic form with the pretreated chipsis rapidly reacted with sodium hydroxide to form a regular kraft cookingliquor of sodium hydroxide and sodium sulfide.

The carbon dioxide from lime kiln 38 can be directed to theprecarbonator 42 and then to gas purifier 21, whence it is directed intopretreatment vessel 10 or into the gas flashing system 30. Nitrogen andother inert gases have to be removed from the lime kiln gas streambefore it enters the pretreatment vessel. As an alternative, externalpure carbon dioxide can be used to provide the gas makeup requirements.

As an alternative, at least a portion of the spent pretreatment liquorcan be recycled back into pretreatment vessel 10, as indicated at 41.

Since the hydrogen sulfide pretreatment system of this invention can bereadily incorporated into an existing kraft mill with no additionalexternal chemical makeup requirements over those normally employed, asillus trated in FIG. 1, there is no net change in the sulfur and sodiumbalances of the kraft m-ill. It should be mentioned that there should bean excess of carbon dioxide in the pretreatment vessel, and this excessacts as a buffer. With continued application of thehigh sulfidity greenliquor and recirculation of recovered hydrogen sulfide and carbondioxide in a continuous system, the partial hydrogen sulfide pressurebuilds up to a level sufiicient for carbohydrate stabilization.

If an external supply of hydrogen sulfide is to be avoided, using a highsulfidity liquor such as green liquor and a hydrogen sulfide-carbondioxide gas mixture, then the pretreatment process should be operatedwithin fairly narrow limits of temperature and pressure to obtainmaximum yield increases. It has been found that the maximum operatingtemperature is about C. for Western hemlock wood chips, above thistemperature pulping via acid hydrolysis will occur, and this leads toweaker pulps,

However, with other species the temperature can be up to about 165 C.and as low as about 120 C. As the temperature decreases to below about130 C., a hydrogen sulfide pressure of 80 p.s.i. or greater is requiredto obtain maximum yield increases with this species. An equilibriummixture of approximately 4 to 7 parts carbon dioxide and about 1 parthydrogen sulfide results in continuous pretreatment of lignocellulosicmaterial in the manner described herein. For a hydrogen sulcfidepressure 20 p.s.i. at 140 C., the total operating pressure should beabout 180 p.s.i., including steam pressure. The preferred operatingconditions for Western hemlock wood chips, are from about 135 to about145 C at a total pressure of from about 120 to about 250 p.s.i. forabout 20 to about 200 minutes. The sulfur application to thepretreatment in the form of sodium, potassium, or ammonium sulfide isbetween about 2.5 and about 5.0 as percent sulfur on wood. About 2.5% assulfur on wood is derived from the smelt and the balance is derived fromthe hydrogen sulfide in the recycled gas which can build up to a levelof between about 1 and 2% as sulfur on wood, from which partialpressures in excess of 30 p.s.i. at 140 C. can be obtained. The carbondioxide makeup requirement depends on the sulfur application in thepretreatment stage and the hydrogen sulfide consumption, as organicsulfur on the lignocellulosic material, during pretreatment. The makeupcarbon dioxide is normally added to an extent of about 2 to 4% on wood,to maintain a total pressure of about 160 to 185 p.s.i. in thepretreatment vessel.

Both the pretreatment and the subsequent pulping stages can be operatedas a 'batch or a continuous system.

Of the various alternative methods of routing the spent pretreatmentliquor, as indicated in FIG. 1, the best choice will depend on theoptimum operating conditions of the pretreatment in any particular mill.This will depend pri-marily on wood species and its response to theprocess in terms of sulfur uptake and yield increase, and also thesulfur available in any particular green liquor system, and the limecapacity of the mill. Generally, if the sulfur application requirementfor the pretreatment from the green liquor is less than, or equal tothat normally available in the green liquor system, then the spentpretreatment liquor may be added to the dissolving tank 35, whiteliquor, or black liquor, depending on factors such as the lime capacityand etc. of the mill. If the sulfur application is greater than thatnormally available in the green liquor, then some recycling of the spentpretreatment liquor must be made, either to the smelt tank, or to thegreen liquor ahead of the crystallizer evaporator, or to the highsulfidity green liquor stream, or directly to the pretreatment vessel,or to the wash zone of the pretreatment vessel after degassing.

Summing up, the present process for pretreating lignocellulosic materialwith hydrogen sulfide generated in the pretreatment by mixing acompatible high sulfidity liquor and an excess of carbon dioxide gas isconducted generally under the following conditions:

Temperature 120 to 165 C. Time (at temp.) 20 to 200 minutes. Partialpressure of H 8 10 to p.s.i. Total pressure 120 to 250 p.s.i. Sulfidityof liquor 75 to 200%. Weight ratio of sulfur to sulfur plus sodium inliquor 0.35 to 0.58.

The present invention is further illustrated by the following examples,which should not be construed to limit the operating limits for theprocess. In every case, commercial Western hemlock wood chips fromdifferent wood lots have been used, however the process is equallyapplicable to other wood species and plant material. The pretreatedchips were subsequently digested by kraft liquors.

EXAMPLE 1 A series of pretreatments followed by kraft cooks were carriedout in a 2.5 cu. ft. stationary digester with forced liquor circulation,under conditions as detailed in Table I below. In this series ofexperiments the pretreatment conditions were established with H S andNaSH as active chemicals, together with inactive N gas.

TABLE I Trial A B C D E Pretreatment conditions:

Time to temperature, min. None 30 25 25 2 5 Time at temperature, minNone 110 110 110 Temperature, O None 145 145 145 140 His partialpressure at 140 C., p.s.i None 25 50 75 60 Total pressure, p.s.i None150 150 150 150 NazO application, percent on wood None 2 2 2 1 Percentsulfur uptake by chips None 1. 25 1. 62 1. 98 1. 90 Results ofsubsequent kraft pulping:

Screened yield, percent on wood 48. 3 48. 70 48.30 50. 60 Percent136180135 0. 26 0. 40 0. 30 0. 20 0. 50 Permanganate number p. o. 25. 325. 0 22.0 19. 8 26. 9 Screened yield at 22 p. No 42. 8 47. 3 48. 7 48.8 49. 1 Screened yield increase at These results show that a pulp yieldincrease of at least 5% can be expected using pretreatment conditions ofabout 145 C. for minutes with a hydrogen sulfide partial pressure ofabout 40 p.s.i. only.

EXAMPLE 2 In a second series of experiments using a different chip lot,the active chemicals were provided by a simulated high sulfidity greenliquor, essentially an aqueous solution of sodium sulfide, together withcarbon dioxide gas and a portion of H 5 gas, to simulate H 8 gasrecovery and recycling in a continuous system. The conditions andresults are shown in Table II.

TABLE II Trial A B C D E F Pretreatment conditions;

HzS applied, as percent S on wood.... None None 1.91 3.22 3. 88 5.54 Nas apphed, as percent S on wood... None None 2.96 3. 12 3. 09 3. 09NaiCO; applied, as percent NazCO on wood None None 0. 44 0. 42 0. 430.44 Time to temperature, min. None None 31 30 30 35 Time attemperature, min. None None 103 104 104 104 Temperature, C None None 145145 145 145 HiS partial pressure at C None None 29 35 51 52 C02 applied,percent on wood as 002.. None None 10. 0 8.0 8. O 6. 0 Results ofsubsequent kraft pulping;

Screened yield, percent on wood 43. 4 44.4 47. 4 48. 2 48. 48. 3 Percentrejects 0.3 0. 5 0 0. 2 0. 2 0. 1 Permanganate number (p. No 21.5 24.121.8 22.1 22.4 20.2 Screened yield at 22 p. No-.. 43. 6 43. 4 47.5 48.248. 0 48.9 Screened yield increase at 22 p. No 4.0 4. 7 5. 1 5. 4

These experiments show that with an H 5 partial pressure of 40-50p.s.-i., about a 5% yield increase is possible under these pretreatmentconditions. In this instance the pretreatments were carried out underconditions closely simulating those anticipated under continuousoperation, with high sulfidity liquor and a CO /H S gas phase.

EXAMPLE 3 Having established that good yield increase are possiblewithlow H S partial pressures in the presence of high sulfidity liquorsand CO gas, a continuous process simulation was carried out. In thissimulation, wood chips were pretreated with high sulfidity green liquorand CO; gas only, after which the gas was recovered by flashing andcompressing into a storage tank. The spent pretreatment liquor wasdrained from the chips into a storage tank, and the chips were washedbriefly with a small amount of water and CO: to simulate the operationof the washing zone. The wash liquor was then drained from the chips andadded to the stored pretreatment liquor. This entire liquor sample wasthen flashed to recover the last traces of free H S or CD which wereadded to the gas storage tank. The spent liquor was removed from thesystem. The chips were then pulped in the usual manner with a lowsulfidity kraft liquor.

In successive pretreatments fresh high sulfidity green liquor was used;together with the recovered gas from the previous pretreatment andmakeup CO: to 185 p.s.i.g. at 140 C. At no time was any external H 8added to the system. Six sucessive pretreatments were carried out usingthe same recycled gas throughout. Pretreatment conditions were 140 C.for 110 minutes in each case. The results were shown in Table 111 usinga third Western hemlock chip lot.

ratio in the feed liquor, less chemical as percent S on wood is requiredto generate the same partial pressure (or overpressure) of hydrogensulfide. For Western hemlock chips, the relation between hydrogensulfide partial pressure under similar pretreatment conditions andscreened pulp yield increase, as shown in FIG. 4. These results wereobtained using a batch laboratory digester with forced liquorcirculation. At a hydrogen sulfide partial pressure of 25 p.s.i., a 5.5%yield increase is attained. With a feed liquor containing sodium sulfideonly S m: about 0.35,

a 6% application as sulfur on wood is required. However, with a feedliquor containing only sodium hydrosulfide =about 0.58,

only a 2.8% application of sulfur is required, FIG. 3. Feed liquorscontaining a proportion of both sodium sul- TABLE III A B o D E F HPretreatment conditions;

Percent onwoodin greenliquormakeup.. None None 4.8 4.8 4.8 3.4 3.4 4.0Percent-Supplied from gas storage None None 0 0.85 1.01 1.32 1.12 1.02Totals applied, percent on wood None None 4.8 5.6 5.8 4.7 4.5 5.0 CO:makeup, percent on wood.. None None 12.0 5.0 3.3 3.3 3.1 4.3 00:applied, percent on wood.. None None 12.0 15.6 15.8 14.2 13.6 14.4Sulfur recovery, percent 8 on wood in spent liquor- None None 1.52 1.481.88 1.03 1.39 1.57 Perc'entSon wood ingas storage-. None None 0.85 1.011.32 1.12 1.02 1.33 Totalsrecovered None None 2.37 2. 49 3.20 2.15 2. 412.09 Percent S carried over to kratt None None 2. 4 3.1 2.6 2.5 2. 1 2.1Total pressure (140 C p s 1.. None None 159 179 162 174 167 172 Pulpingre ts;

P.No' 22.3 21.4 24.8 25.1 25.1 22.9 26.3 24.2 Total eld 44.9 44.0 49.750.6 49.7 50.0 50.0 49.3 Screened yield 43.9 44.2 48.2 50.4 40.6 49.350.0 49.2 31 yield at 22 1%.... 44.7 44.8 48.9 49.7 48.9 49.7 48.0 48.6Screened yield at 22 p. N0 43.7 4 .0 47.4 49.5 48.7 49.0 49.0 48.5Screened Yield increase at 22 p. No 3.5 5.6 4.9 5.1 5. 1 4.6

These results clearly demonstrate that a hydrogen sulfide and sodiumhydrosulfide would normally be obfide partial pressure can be built upand sustained in a tamed with a green liquor precarbonation process. Anycontinuous system, without any external H 8 addition, excess sodiumcarbonate or bicarbonate in the feed liquor sufiicient to give excellentpulp yield increases over reguwould decrease the lat kraft cooks. Inaddition, the sulfur carryover to the kraft stage can be kept at orbelow 3% on wood. Close insepection of these results shows that theamount of H 8 S recovered is controlled by the sulfur application from thigh sulfidity green liquor.

EXAMPLE 4 ratio. High In this case the process simulations were carriedout on a 15 ton per day continuous pilot plant, consisting of apretreatment vessel and digester of similar design S and operatingprinciple to that commonly used in full size S+N commercialinstallations. Western hemlock wood chips were used as feed material;feed chemicals for the pretreatment vessel were prepared from sodiumhydrosulvalues for the feed liquor are prepared by precarbonating fideand sodium hydroxide solutions; carbon dioxide for regular green liquorfrom a pH about 13.5 to a pH of the pretreatment was supplied from astorage tank; regular about 12, whereby higher concentrations of NaSHare mill cooking liquor (white liquor) was used in the dlgester.generated, see FIG. 2, prior to the crystallization/evap- Feed liquorsof different chemical composition were oration step. Impure carbondioxide can be used for prepared and feed continuously to thepretreatment ves- 4 the precarbonation process. This could be obtaineddirectsel, in which the pressure was maintained at 180 psig. ly from thelime kiln or recovery boiler of a kraft mill.

TABLE IV The efleet of feed liquor composition and application on thegenerated H23 partial pressure and CO1 consumption in a continuous chippretreatment system with continuous gas recycling C02 consump- Feedhquor Generated tion at equi- Feed liquor composiapplications H28partial librium as tion as weight ratio of as percent pressure atpercent on 8/8 Na S on wood equilibrium wood 0.58 2. 9 28. 2 1. 8 0.582. 9 28. 2 2. 6 0.58 2. 4 21. 2 1. 7 0.58 5. 45. 8 3. 0 0 .58 2. 6 2. 00.41 2. 5 13.0 3. 2 0.41 3. 5 18. 0 3. 7 0.58 5. 0 50. 2 0.41 5. 8 18. 66. 8 0.47 5. 0 21. 1 0.58 3. 1 2.0 0.50 3. 7 26. 5 5. 4 0.45 3. 7 21. 33. 6 0.58 2. 9 27. 4 2. 0 0.43 5. 6 26. 1 8. 7 0.50 3. 0 23. 1 2. 4 0.413. 0 l4. 9 3. 8 0.46 5.6 31.9 5.4 0.54 5.6 50.0 2.3 0.41 3. 7 19. 0 4. 60.54 3.7 39.7 2. 6 0.33 l. 8 9.4 5.1 0.42 1.8 13. 8 2. 3 0.50 4. 3 29. 44. 8 0.44 3. 0 22. 2 4. 6 0.38 3. 0 15. 2 8. 0 0.36 5.4 21. 0 11. l 0.374. 6 18. 6 12. 2 0.36 3. 7 13. 5 8. 9

TABLE V Trial A B C D E Pretreatment conditions:

Time at temperature, min.. None 50 50 50 50 Temperature, C None 140 145150 155 HaS partial pressure at 140 p.s.i None 17. 4 12. 9 14. 5 12. 8Total pressure, p.s.i None 180 180 180 180 Results after subsequentkrait pulping:

Screened yield, percent on wood 43.9 48.0 48.0 Percent rejects 0. 7 0. 50. 3 2 Permenganate number (p.

No. 23. 9 21. 3 21. 5 20. 3 22. 4 Screened yield at 22 p. No.. 43. 3 48.2 48. 2 46. 5 40. 8 Screened yield increase at 22 p. No 4.9 9 3 Theseresults show that increasing the temperature to 150 C. or above reducesthe yield increase with this wood species. The preferred temperatureconditions for the pretreatment is between 140 and 145 C.

EXAMPLE 6 A series of chip pretreatments followed by kraft cooks werecarried out as in Example 2, but in this instance, the pretreatment timewas varied. The results shown in Table VI show that with Western hemlockchips about 50 minutes is the minimum time requirement for the mosteffective pretreatment.

TABLE VI Trial A B C D E F Pretreatment conditions:

Time at temperature, min None Temperature, C None 138 138 138 138 138 aspartial pressure at 140 p.s.i. None 20. 2 17. 5 16.4 23. 8 18. 6 Totalpressure, p.s.i None 156 156 172 177 Results after subsequent kraitpulping:

Screened yield, percent on wood 42.6 47.3 47.1 48. 0 47.0 47.1 Percentrejects 0.8 0.7 0. 8 0. 5 0.2 0. Permanganate number (p. No.) 22.6 26.225.0 26.2 23.3 24.0 Screened yield at 22 p. N o 42.4 45.9 46. 1 46. 646.6 46.4 Screened yield increase at 22 p. No 3. 5 3. 7 4. 2 4. 2 4. 0

Consumption of carbon dioxide while operating the 'What is claimed is:

continuous pilot plant under conditions described above, is shown inFIG. 5 and Table IV. It is evident that by the presence of sodiumhydrosulfide in the pretreatment feed liquor (higher ratios), less puremakeup carbon dioxide to the pretreatment vessel is required.

in a 2.5 cu. ft. stationary digester with forced liquor circulationunder conditions as detailed in Table V.

1. In an alkaline pulping process in which lignocellulosic material ispre-treated with hydrogen sulfide, in which the pro-treated material isdigested under heat and pressure in an alkaline digesting liquorcomprising a sodium based digesting agent to form a pulp, in which thedigested pulp is separated from the digesting liquor after digestion,and in which the digesting liquor is processed for recovery of thealkaline digesting agent, the improvement which comprises:

providing from the digesting liquor processed for recovery of thealkaline digesting agent, a high sulfidity liquor containing at leastone member selected from the group consisting of sodium sulfide andsodium hydrosulfide and having a sulfidity of about 75% to 200% and aweight ratio of sulfur to sulfur plus sodium of from about 0.35-0.58;

contacting the lignocellulosic material with said high sulfidity liquorin a pre-treatment vessel proir to alkaline digestion;

reaction between hydrogen sulfide and the lignocel lulosic material insaid pre-treatment vessel to increase pulp yield over that obtained byan alkaline pulping process in which the iignocellulosic material isdigested in said alkaline digesting liquor;

separating pro-treated lignocellulosic material from the pre-treatrnentliquor present in said pre-treatment vessel;

separating hydrogen sulfide gas from the separated pre-treatment liquor;and

recycling at least a portion of said hydrogen sulfide gas separated fromsaid pre-treatment liquor to said pretreatment vesselfor treatment ofadditional lignocellulosic material introduced thereto, whereby the needfor adding make-up hydrogen sulfide for said pretreatment from anexternal source is eliminated.

2. A process according to claim 1 wherein the lignm cellulosic materialcomprises wood chips.

3. The process according to claim 1 wherein the pH of the liquor in thepre-treatment vessel is maintained in the neutral range.

4. The process according to claim 1 wherein the total pressure in thepre-treatment vessel ranges from about 120 to 250 p.s.i.

5. The process according to claim 1 wherein the pretreatment liquorseparated from said lignocellulosic material is introduced into thedigesting liquor recovery system of the alkaline pulping process.

6. The process according to claim 1 wherein the processing of thealkaline digesting liquor includes the formation of a smelt and whereinthe pre-treatment liquor separated from the lignocellulosic material isused for dissolution of said smelt.

7. The process according to claim 1 in which the alkaline pulpingprocess is the kraft pulping process.

8. A method as in claim 1 in which the high sulfidity feed liquorsolution contains sodium carbonate, sodium bicarbonate and sodiumhydroxide.

9. A method as in claim 7 in which processing of the kraft digestingliquor includes formation of a green liquor and in which said highsulfidity liquor is obtained from said green liquor.

10. A method according to claim 1-wherein hydrogen sulfide gas isseparated from the. separated pre-tratmen liquor in the presence ofcarbon dioxide gas.

References Cited 4 UNITED STATES PATENTS S. LEON BASHORE, PrimaryExaminer A. L. CORBIN, Assistant Examiner US. Cl. X.R.

1. IN AN ALKALINE PULPING PROCESS IN WHICH LIGNOCELLULOSIC MATERIAL ISPRE-TREATED WITH HYDROGEN SULFIDE, IN WHICH THE PRE-TREATED MATERIAL ISDIGESTED UNDER HEAT AND PRESSURE IN AN ALKALINE DIGESTING LIQUORCOMPRISING A SODIUM BASED DIGESTING AGENT TO FORM THE PULP, IN WHICH THEDIGESTED PULP IS SEPARATED FROM THE DIGESTING LIQUOR AFTER DIGESTION,AND IN WHICH THE DIGESTING LIQUOR IS PROCESSED FOR RECOVERY OF THEALKALINE DIGESTING AGENT, THE IMPROVEMENT WHICH COMPRISES: PROVIDINGFROM THE DIGESTING LIQUOR PROCESSED FOR RECOVERY OF THE ALKALINEDIGESTING AGENT, A HIGH SULFIDITY LIQUOR CONTAINING AT LEAST ONE MEMBERSELECTED FROM THE GROUP CONSISTING OF SODIUM SULFIDE AND SODIUMHYDROSULFIDE AND HAVING A SULFIDITY OF ABOUT 75% TO 200% AND A WEIGHTRATIO OF SULFUR TO SULFUR PLUS SODIUM OF FROM ABOUT 0.35-0.58;CONTACTING THE LIGNOCELLULOSIC MATERIAL WITH SAID HIGH SUFIDITY LIQUORIN A PRE-TREATMENT VESSEL PRIOR TO ALKALINE DIGESTION; INTRODUCINGCARBON DIOXIDE GAS INTO SAID HIGH SULFIDITY LIQUOR IN SAID VESSEL, IN ANAMOUNT IN EXCESS OF STOICHIOMETRIC TO GENERATE HYDROGEN SULFIDE FROMSAID HIGH SULFIDITY LIQUOR; MAINTAINING THE TEMPERATURE IN SAIDPRE-TREATMENT VESSEL WITHIN THE RANGE OF ABOUT 120 TO 165*C. FOR ABOUT20 TO 200 MINUTES AND UNDER A PARTIAL PRESSURE OF HYDROGEN SULFIDE GASOF ABOUT 10 TO 80 P.S.I. TO EFFECT REACTION BETWEEN HYDROGEN SULFIDE ANDTHE LIGNOCELLULOSIC MATERIAL IN SAID PRE-TREATMENT VESSEL TO INCREASEPULP YIELD OVER THAT OBTAINED BY AN ALKALINE PULPING PROCESS IN WHICHTHE LIGNOCELLULOSIC MATERIAL IS DIGESTED IN SAID ALKALINE DIGESTINGLIQUOR; SEPARATING PRE-TREATED LIGNOCELLULOSIC MATERIAL FROM THEACCORDING TO WHICH THE SUPPORT TUBES PASS THROUGH THE BOTTOM OF THEREACTOR VESSEL AND ARE PROVIDED AT THE OUTER END WITH AT LEAST ONE PLUGWHICH ACTS AS A RADIATION SHIELD AND A COOLING-GAS SEAL, IN WHICH THESUPPORT TUBE (8) IS FIXED IN A SLEEVE TUBE (3) BY MEANS OF A LOCKINGPLATE (5, 6) SUCH THAT BETWEEN THE SUPPORT TUBE (8) AND THE SLEEVE TUBE(3) THERE IS AN ANNULAR GAP (11) CONTAINING AN AUXILIARY TUBE (12) WHICHIS MOVABLE IN THE AXIAL